I'm using water heat now (since last weekend) and it doesn't take a whole lot of time to heat the intake. 5 minutes on the outside. I haven't actually measured the temp, but it isn't going to lose a whole lot of temp in 24" of rubber hose. When I had exhaust heat, the flapper never worked for 30 years and I didn't even know it was there for most of that time, I wedged into the heat on position once, dear lord I thought it was going to explode. Heating it too much isn't good either.
